Accessible Design in Open Source

Design is not simply skin deep, it interacts with deeper layers in software programs. And it is not just for people who point and click to use applications. It also affects people who cannot distinguish certain colours, who cannot use a mouse, who are confused by complex user interfaces, etc.
What can designers and developers in open source projects do to make their software more accessible? What do they need to know about people with disabilities? What tools are available to help them? What are example projects that have worked to improve accessibility?